A number of circuits have evolved which take advantage of the ability of field effect transistors to store charge and thus serve as memory cells. Such cells may be either dynamic or static in nature. The dynamic cells may employ only a single field effect transistor and the static cells may be arranged in a flip-flop configuration, as is well known. Each of these types of cells may be referred to as volatile cells since information stored in these cells is lost when the power supply voltage applied to the memory is lost or turned off. In instances where stored volatile information must be retained, an alternate power supply, such as a battery system, must be coupled to the memory for use in the event of failure of the main power supply.
Known devices capable of providing variable threshold voltages, such as field effect transistors having metal-nitride-oxide-silicon (MNOS) and field effect transistors having a floating gate are capable of storing information in a non-volatile manner for long periods of time. By incorporating such non-volatile devices into memory cells, there has been provided normally operating volatile cells which do not require a backup or alternate power supply for preserving information when power interruption or failure occurs in the main power supply.
The non-volatile memory cells which use non-volatile MNOS transistors or devices are capable of retaining for long periods of time information stored volatilely in a cell but these devices require high voltage pulses for writing and erasing the information, they are slow and they require rather complex processes for their fabrication. An example of a non-volatile semiconductor memory cell is taught in U.S. Pat. No. 3,676,717, filed Nov. 2, 1970.
Known non-volatile memory cells which use conventionally arranged floating gate devices are also capable of preserving for long periods of time information stored volatilely in a cell but these devices likewise have required high voltage pulses for writing and erasing the information, they have been slow and required high currents, approximately one milliampere per device, to write. An example of a known non-volatile semiconductor memory cell having incorporated therein a floating gate is taught in U.S. Pat. No. 4,207,615, filed Nov. 17, 1978.
In commonly assigned U.S. patent application Ser. No. 192,579, filed on Sept. 30, 1980, now U.S. Pat. No. 4,388,704, C. L. Bertin, H. N. Kotecha and F. W. Wiedman, now U.S. Pat. No. 4,388,704, there is disclosed non-volatile static memories which include a volatile circuit coupled to a non-volatile device having a floating gate and first and second control gates capacitively coupled to the floating gate with a charge injector structure including enhanced conduction insulators disposed between the floating gate and one of the two control gates. A detailed discussion of enhanced conduction insulators may be found in an article entitled "High Current Injection Into SiO.sub.2 from Si rich SiO.sub.2 Films and Experimental Applications" by D. J. DiMaria and D. W. Dong, Journal of Applied Physics 51 (5), May 1980, pp. 2722-2735, and a basic memory cell which utilizes the dual electron injector structure is taught in an article entitled "Electrically-Alterable Memory Using A Dual Electron Injector Structure" by D. J. DiMaria, K. M. DeMeyer and D. W. Dong, IEEE Electron Device Letters, Vol. EDL-1, No. 9, September 1980, pp. 179-181.
Highly dense dynamic random access memory (RAM) cells having only a single storage capacitor and a single switch or transistor are disclosed in commonly assigned U.S. Pat. Nos. 3,387,286, filmed on July 14, 1967, by R. H. Dennard and 3,811,076, filed on Jan. 2, 1973, by W. M. Smith.
One device dynamic volatile memory cells having the capability of storing data non-volatilely are known. For example, commonly assigned U.S. Pat. No. 3,916,390, filed Dec. 31, 1974, by J. J. Chang and R. A. Kenyon discloses the use of a dual insulator made of silicon dioxide and silicon nitride for storing information non-volatilely during power failure. Other examples of dynamic cells capable of storing non-volatilely by using MNOS structures include U.S. Pat. Nos. 4,055,837, filed Oct. 22, 1975, by K. U. Stein et al and 4,175,291, filed Oct. 31, 1977, by W. Spence. These dynamic cells having non-volatile capability can operate satisfactorily, however, they generally required both negative and positive voltages to switch between volatile and non-volatile modes, have larger cell areas, and have larger voltages for the volatile operating mode or a backup memory.